An airplane and a conveyor belt.

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction).

Can the plane take off?

Ned
February 11th, 2006 10:26am

Doubt it.The conveyor can provide the required thrust, but probably not the required lift. So you might require a conveyor placed at an angle to the ground.

It'll take off, although some tire/wheel damage may occur from the wheels moving twice as fast as expected. The reason is like Flasher says -- the planes forward speed is dependant the engines pushing against the air, and not the wheels pushing against the runway/conveyor like in a car.

Oh, and I located the pics of the Jet Blue flight that landed with the front gear pointing the wrong way. The concrete runway ground the wheel down to the hub.

the wheels on aircraft are unpowered - they freewheel. So the conveyor doesn't really have any effect on anything - the aircraft propels itself with its engines and it's all about airspeed.

Philo

Philo
February 11th, 2006 11:17am

Exactly Philo.
You'll need a realy fast conveyor belt to get the aircraft to a suitable speed where it can take off without any internally provided force.
It's the same thing that happens to racing drag cars when they gain speed of that monstrosity. They take off !

I'm gonna say that Necromancer is the only one answering the actual question.

It's not getting lift, so it can't take off.

Essentially, I think it is thrusting like Flasher is saying, but it's in a stall, so it can't take off from the ground.

I think Philo is missing that it's a theoretical question, about the wheels.

LinuxOrBust
February 11th, 2006 11:47am

I don't think anyone other than Aaron understood the question... it sounds like he's set it up so the plane is stationary relative to the ground. It's like walking the wrong way on an escalator. If you walk at the same speed as the "belt" you don't move relative to the ground.

So in this case the plane isn't moving relative to the ground, which means no lift.

Ward
February 11th, 2006 11:53am

If you want to support Aaron, you should quote him:

"A conveyer belt will not move the air over the wings.

Now, if you had a hellacious fan..."

So if you had a hellacious wind blowing against the wings, from the front, it would take off.

LinuxOrBust
February 11th, 2006 12:02pm

The propellors/jets are not going to be moving air over/below the wings, right? They essentially move the plane through otherwise stationary air which generates lift, as the surrounding air is moving relative to the plane.

Now, if the propellors are blowing enough air over/below the wings, sure, you'll get lift.

Which is why I said you need a big ass fan.

Aaron F Stanton
February 11th, 2006 12:05pm

Of course, the forward motion of the plane is independent of the wheels - otherwise flight is impossible.

I was under the impression that something was holding it in place, but since it wasn't stated, I am probably wrong.

Aaron F Stanton
February 11th, 2006 12:07pm

Here's another fun one:

Let's say you have a massive wheel, large enough to have appreciable gravity. Let's say it's rotating fast enough that the force at it's outer edge exactly balances it. What happens if you are standing on the outer edge and jump?

Okay, seriously - you guys all know that airplanes have no propulsion system other than the engines, right? The wheels have nothing to do with moving the thing - it's all engine.

The equation is about thrust on the fuselage vs. drag - the wheels only hold the thing off the ground until it's got enough lift to take off. You'll note that arctic aircraft have skis.

So that there's a conveyor moving backwards just means the wheels are spinning faster (and perhaps getting more drag). But the aircraft, propelled by the engines on the fuselage, is still going to move forward. And generate lift. And take off.

Philo

Philo
February 11th, 2006 12:18pm

Philo, did you miss the bit where this hypothetical wonder-belt tracks and adjusts to the speed of the airplane to ensure that it stays stationary with regard to the landscape?

Hence no forward movement (with regard to the air), thus no lift, thus no take-off.

Or did I miss something?

Jp
February 11th, 2006 12:27pm

>> What happens if you are standing on the outer edge and jump? <<

Are you standing on the inside of the wheel or the outside?

(My answers are assuming a "ringworld" situation, and not a "ferris wheel" -- the scale and any outside forces matter.)

Inside: the jump works more or less as expected, taking the outward force imparted by the rotating wheel, it's gravity, and some coriolis force.

Outside: with gravity being balanced by the spin, you aren't really attached to the wheel except by some horizontal friction. Asumming you manage to execute a jump anway, what happens depends on whether you exceed the escape velocity caused by the wheel's gravity.

And since the wheels on the plane are freewheeling, how does the belt impart any resistance to the plane's movement?

example
February 11th, 2006 12:30pm

ARgh.

The wheels are free-wheeling. How is the conveyor going to hold it back?

Think again - aircraft on skis on a frictionless surface. How do you explain that?

How fast the ground is moving, in any direction, has nothing to do with the airspeed of the aircraft. The wheels just keep it off the ground.

Philo

Philo
February 11th, 2006 12:32pm

*brain switches on*

Ta for flicking the switch...

Jp
February 11th, 2006 12:34pm

Jp:

The conveyor doesn't push the airplane back. It simply matches the plane's airspeed. The plane's propulsion does not occur on the ground, it occurs in the air.

Aaron:

If the centrifugal force balances out the gravity, then what you have at the edge of the wheel is weightlessness. Then again, once you stop being subject to the centrifugal force, you lose the countervector and the gravity is not balanced any more.

Wow. A larger part of the world's stupidity than I thought possible has manifested itself in this thread. We're doomed. Doomed I tell you.

Ian Boys
February 11th, 2006 2:09pm

The problem is still that Ned didn't ask the question clearly.

Put a plane on a conveyer belt and run the belt backwards - the plane moves backward. Don't get into high speed, just picture a plane on a giant movator like they have in most airport.

Start up the plane's engines and give a bit of thrust, and
the plane stops moving backward.

Sure, if the question means frictionless wheels in addition to a giant conveyor belt, then it doesn't matter what the belt does.

In fact, if it's frictionless wheels, then you put the plane on the belt and turn it on, and the plane just sits there, since there's no friction to transfer force to the plane.

Ward
February 11th, 2006 4:00pm

Or how about this classic:

Take an infinately long, very dense cylinder and spin it so that the rim is moving at close to lightspeed. What happens to space-time in the vicinity?

Google for: tippler cylinder

Ward
February 11th, 2006 4:02pm

HEY WARD . YOU LITTLE SHITHEAD.

WEREN'T YOU THE ONE WHO WAS GOING TO POST IN ALL CAPS

UNTIL I CHANGED MY POSTING NAME?

Kasey
February 11th, 2006 4:45pm

Ian,

You mean PhD could make mistake? :)

Rick Tang
February 11th, 2006 5:05pm

*Go to a treadmill and try*

Rick Tang
February 11th, 2006 5:17pm

I looked it up on Wikipedia, Tipler's Cylinder:

"A limitation of the Tipler Cylinder is that it is only possible to travel to times (and places) in which the cylinder already exists. Thus, one could not travel backwards further than the date that the cylinder was activated, nor beyond a point after which the cylinder was destroyed (assuming such destruction is even possible). One could hypothesize that this is why humans have not yet encountered a time traveler."

That's pretty funny.

LinuxOrBust
February 11th, 2006 7:01pm

So the day it's built, we'll get a flood of time-traveling tourists.

Philo
February 11th, 2006 7:10pm

The other limitation of the Tipler cylinder is that it requires an infinite cylinder - Tipler's calculation was based on an infinitely long cylinder and _speculated_ that the solution might be the same for a very long one if you stayed away from the edges.

Kasey: 19 34a

Ward
February 11th, 2006 7:32pm

Hey Wardo, you read the wikipedia entry stop pretending like you knew the man.

Kasey
February 11th, 2006 7:48pm

Hey Kasey, I know _you_ can't read, but in a forum full of geeks, is it so surprising that some of us read science fiction? Anyone who's read the short story with the same title as Tipler's paper would be able to track the paper down in PhysRevD.

Ward
February 11th, 2006 7:57pm

You went to the library and picked up a 1974 copy of the article, you paid $23 bucks for at aps.org, or you are cribbing the last sentence of the wikipedia article?

Hmmm. You did not read the original paper in Phys. Rev. D.

Kasey
February 11th, 2006 8:11pm

Since you seem to be someone whose mommy still makes him PB&J sandwiches for lunch, you will be amazed to know that photocopiers existed as far back as the mid-1980's... And since you're obviously not the type to attend university, let me clue you in that, yes, physics undergrads sometime make use of libraries to get copies of articles they don't really understand.

Ward
February 11th, 2006 8:48pm

What is the claim now? That you have a photocopy of the paper handy?

This should be easy to prove, what is the text of footnote 17?

Kasey
February 11th, 2006 8:53pm

At least with No Child Left Behind, you're sure to graduate...

Where exactly do I say I have it handy?

Ward
February 11th, 2006 9:10pm

I'm not sure why you're continuing to embarass yourself.

Now, you are claiming you don't have the paper handy, but remember from photocopying it 20 years ago, that he did the calculations on an infinite cylinder and speculated about finite lengths.

Either that or you are cribbing from wikipedia.

Hmmm.

Perhaps you recall his objections to the causality violiation found in Godel's solutions to the field equations? Or by wild coincidence does your mid-80s physics education correspond exactly with what is printed in the wikipedia?

Kasey
February 11th, 2006 9:23pm

OUCH Ward. Kasey just poked you in the eye with his HUGE academic penis.

- never gone to university so he's never taken part in a college bull session or maybe he did go to university but was such an annoying little prick that no one would talk to him

Ward
February 11th, 2006 9:37pm

University libraries are fun places. While browsing the outer reaches of the bookshelves I once found a copy of Nancy Friday's "My Secret Garden" which, considering I was at a technical university, was somewhat surprising. The explanation was that somehow they had convinced themselves that Social Sciences was a scientific discipline, much to the ridicule of everyone studying real science.

[Note for American readers: British universities don't make you waste time taking arts courses if you are studying a subject like physics. They just provide facilities for arts, music, theatre, sports etc. and expect you to avail yourself of them as extramural activities.]

Ian Boys
February 11th, 2006 9:59pm

So if you're in a car driving the speed of light and you turn on the headlights, what happens?

Aaron F Stanton
February 12th, 2006 12:16am

If I measure the speed of light, I get the standard #.

Duh. I never made it through very much of Misner/Thorne/Wheeler, but hasn't everyone had some SR?

(For Kasey: SR is Special Relativity)

Ward
February 12th, 2006 12:30am

That's cheap shot, Ward.

Rick Tang
February 12th, 2006 12:52am

Can God make a rock that he can't raise?

Rick Tang
February 12th, 2006 1:01am

Just one comment - the rate of velocity over the top of the wing and the bottom of the wing is really overrated.

The vast majority of lift in modern aircraft comes from reaction lift, which is basically just deflection: The wing is angled such that incoming air is deflected downwards, at the same time "pushing" the wing upwards (and back, of course, which is why you need big engines pushing forward). Like sticking your hand out of the car window at different angles so it pushes your hand up or down.

The Bernoulli effect is quite real. I imagine that in level flight it predominates, but if someone here is a pilot...

Ward
February 12th, 2006 1:36am

I didn't say it wasn't real. I said it was overrated. On large aircraft it accounts for less than 1/5th the lift. Feel free to do a web search - there are countless articles on this. It's just a really nice and easy to fathom principal, so it gets taught as the foundation of flight, when it actually isn't.

If we discount any downward thrust from engines, then *all* the lift on an airplane comes from downward deflection of the airstream. This follows directly from Newton's second and third laws of motion. In level flight the downward vertical component of the total mass acceleration of the air caused by the wing must exactly equal the upward force acting on the wing. This is true because the air is a free fluid and there is no other way for it to exert an unequal force on a body contained within it other than by being accelerated by that body, and there is nothing other than the air that can exert a force on the wing. (The air is deflected downwards by a complex mechanism that may be explained in more than one way, and has to be calculated by solving the flow equations for the wing geometry.)

It must also be true that the vertical component of the integrated pressure difference over the total upper and lower surfaces of the wing will equal the upward force acting on the wing. The fact that the calculated lift from both approaches must be exactly equal, in spite of the entirely different methods of arriving at them, is one of those beautifully elegant results that sometimes arises in science and engineering.

If anyone should doubt that wings deflect air downwards, just try standing under a helicopter some time, which is in fact no more than a set of wings rotating on a stick.

Ian Boys
February 12th, 2006 2:43am

"What disagreements, Sir?"

Just look at the reviews of the book I linked above. If you've paid attention to this sort of thing, there has been a lot of controversy, and there still are a fringe number of believers that it's all Bernoulli's effect.

Get your Airport Runway Conveyor Belt (TM) right here folks, only $999,999.99. Order now while supplies last. No land for that new airport you say? Build it right here on 2 acres! Planes can land on it too!

Born Seeker
February 12th, 2006 1:05pm

Ward, you know better...

You can't go the speed of light unless you're massless, so the question itself is meaningless.

Aaron F Stanton
February 12th, 2006 9:36pm

Hm. Did Aaron read this earlier, or was this just a really bad day to say that?